[ashafer]

iirc it's pretty much completely up to the compositor. The spec is worded to allow "focus" to enter a surface and deliver events to it, but it never specifies under what conditions the compositor should do so.

So the compositor chooses what surface to deliver events to based on its own desires (like letting pointer focus enter background apps) and the users input. I think there is a protocol (used by Xwayland?) to allow a client to get events from any window if the compositor/user allows it

[Cloudef]

That's how it is. It's up to compositor to decide what events to send to what clients. Usually this means compositor only forwards input events to the focused surface.